The short answer
Classic sudoku is a game of pure placement: you are handed a scatter of digits and you complete the grid by logic alone. Killer sudoku keeps that grid and its rules but hides almost all of the starting numbers, replacing them with dotted cages that each carry a target sum. The result is that classic sudoku rewards patient positional reasoning, while killer sudoku asks you to reason from arithmetic and position at the same time. Both are complete, single-solution puzzles; they simply hand you different kinds of information to start from.
What the two share
Everything structural is common ground. Both are played on a nine-by-nine grid divided into nine three-by-three boxes. Both obey the same three constraints: every row, every column, and every box must contain the digits 1 to 9 exactly once. Both are guaranteed to have a single solution reachable by deduction, so neither ever requires a guess. If you already know how to place a naked single or spot a hidden single, that skill transfers directly — killer sudoku does not throw away a single classic technique.
What killer sudoku adds
Cages
The board is partitioned into cages: groups of orthogonally connected cells marked by a dotted border. Cages come in all shapes and sizes — a single cell, a straight pair, a bent triple, an L of four — and they can straddle box boundaries and span rows and columns. Where classic sudoku shows you fixed digits, killer sudoku shows you these outlines.
Sums
Each cage carries a small number in its top-left corner: the sum its digits must add up to. A two-cell cage labelled 4 must hold two digits totalling 4. That single number is the payload — it encodes far more than it appears to, because a target sum sharply limits which digits can share a group of cells.
No repeat inside a cage
This is the rule newcomers forget most often. A digit may not repeat within a single cage, even where the row, column, and box rules would allow it. So a two-cell cage summing to 4 cannot be 2 + 2; it must be 1 + 3. That constraint is what makes sums so powerful, because it collapses many sums to a single possible combination of digits.
Few or no givens
A classic puzzle hands you a scatter of given digits. A killer puzzle usually gives you none at all, so it looks intimidatingly blank. The trade is fair: the cages carry the information the givens would have, just in a form you have to decode with arithmetic.
How the solving differs
In classic sudoku the whole game is elimination by position. You scan units for the one cell a digit can occupy, watch for cells with a single remaining candidate, and chain those deductions across the board. The hardest classic puzzles lean on patterns like the X-Wing, swordfish, and unique rectangle, but the currency is always the same: where can this digit legally go?
Killer sudoku runs a second engine alongside that one. Before you can place much, you look at each cage and ask which sets of digits could fill it. Many small cages resolve to a single combination — a two-cell cage summing to 17 can only be {8, 9}, a three-cell cage summing to 6 can only be {1, 2, 3} — and those locked sets immediately rule digits out of the surrounding row, column, and box. Then the rule of 45 comes into play: because each region sums to 45, you can subtract known cage sums to deduce leftover cells, and extend that across region boundaries with innies and outies. You interleave this arithmetic layer with the classic positional logic, switching between the two as each stalls. The full method is laid out in the killer sudoku rules and strategy guide.
How they feel to play
Classic sudoku has a calm, meditative rhythm. The rules are held entirely in your head, and progress is a steady accumulation of placements. It is the puzzle you can pick up for two minutes in a queue and put down without losing your thread.
Killer sudoku is spikier. The opening can feel like staring at a blank wall, and then a single recognised combination cracks a corner open and a run of deductions tumbles out. It rewards a slightly different temperament — one that enjoys mental arithmetic and does not mind a slow start in exchange for a satisfying unlock. Players who like the numbers side of the puzzle tend to find killer more absorbing; players who like pure spatial logic often prefer classic.
Which should you play
If you are new to sudoku, start with classic. It teaches the three rules and the core elimination habits without any arithmetic overhead, and those habits are the foundation killer sudoku builds on. Pick an Easy puzzle, get comfortable with naked and hidden singles, and work up the difficulty ladder from there.
If classic sudoku has started to feel routine, or if you enjoy mental maths, move to killer. It reuses everything you already know and adds a fresh dimension that keeps even experienced solvers thinking. Many players keep both in rotation: classic for a quick, low-friction session, killer for when they want a puzzle to chew on. Sudoku by WizusLabs ships both, plus a range of other variants, so you can switch between them without leaving the app.
Where to play both
Sudoku by WizusLabs generates classic and killer puzzles on device across six difficulties from Easy to Grandmaster, works offline with progress saved locally, and gives every puzzle exactly one solution — so whichever variant you choose, the logic always resolves without guessing. Candidate notes are built in for both.
Keep reading: killer sudoku rules and strategy · sudoku variants explained · sudoku glossary